Typically, a thermode is a device that is commonly used for soldering operations in electronic assembly. The thermode includes a blade supported in a holder and connected to a power supply unit, which includes a transformer. The power supply provides an electric current directly to the blade to heat it during contact with and simultaneous soldering of a plurality of leads of an electronic component to conductor pads on an electronic circuit board.
In order to apply uniform pressure to the component leads and circuit board, the contact surface of the thermode blade should maintain a relatively flat profile. In addition, the blade contact surface should have a uniform temperature so that reliable soldering takes place at all the circuit board pads without overheating any of them. Furthermore, the thermode should maintain these uniformities under various heating and cooling cycles, i.e. heating and cooling that occurs before, during and after contact with the component and circuit board.
When soldering the component to the circuit board, it is often desired to increase the current supplied to the thermode to either reduce the time it takes the blade to attain a desired temperature or to maintain temperature uniformity across the blade. To increase the current through the thermode, a higher voltage is required between the ends of the thermode. Yet, the voltages to which the leads are subjected as a result of contact with the blade must be limited to avoid damage to the component.
The length of the thermode blade is typically equal to the length of the the work surface footprint, i.e. a side of the component having leads to be soldered; typically, the footprint length is approximately one inch. Advances in semiconductor technology have led to components having man leads and, thus, footprint lengths approaching as much as three inches. At the shorter blade lengths, the thermode provides uniform pressure and temperature at the work surface. However, with the longer blade lengths, the thermode blade deflects when contacting the work surface. Such deflection occurs in all blades and is primarily because of (i) thermal gradients in the blade and (ii) mechanical constraint of the blade, which prevents it from expanding freely. The magnitude of the deflection increases with blade length, resulting in uneven pressure and heat distribution at the contact surfaces of relatively long blades.
The performance of a thermode is measured, in part, by its ability to maintain a flat profile in the contact surface of the blade during bonding operations. FIG. 1 shows a prior art, generally U-shaped thermode 60 comprising a blade 62 with left and right side struts 64 and 66. At relatively short blade lengths, e.g. one inch, the U-shaped thermode 60 provides uniform pressure and heat to a work surface. But at longer blade lengths, e.g. three inches, the blade 62 deflects (shown in phantom) when it is heated.
One approach to soldering objects with relatively long footprints has been to use a smaller thermode that can bond a footprint side in sections. For example, a thermode blade first contacts a portion of the leads of a component and soldering ensues. The blade then contacts another portion of the leads to solder them. However, some of the leads may be subjected to two successive soldering cycles, resulting in damage to the component and circuit board. The soldering "overlap" is difficult to avoid because of the close proximity of the leads.
Attempts to increase the length of a thermode blade have resulted in thermode designs incorporating additional power terminals. FIG. 1A depicts such a thermode 70 with a plurality of parallel power terminals 74 oriented perpendicular to the blade 72. The additional power terminals reduce the maximum voltage difference across any section of the blade 72 and thus permit a higher heating current without subjecting components to undue voltage stress. The thermode 70 also provides structural support for a longer thermode blade 72; however, the additional poles 74 tend to create temperature gradients that result in unduly non-uniform heat distribution.
Therefore, it is among the objects to provide a thermode arrangement that minimizes deflection of a relatively long thermode blade during bonding operations.
It is also an object of the invention to provide a thermode with a blade capable of providing uniform heat distribution across a long-footprint work surface.
Additionally, it is an object to provide a thermode arrangement with a structure that provides uniform pressure across the long-footprint work surface.